Question: If $m \blacktriangle n = 2m+n$ and $m \blacktriangledown n = 4m-n$, what is the value of $(2 \blacktriangle (5 \blacktriangledown 3))$?
Solution: We have $5 \blacktriangledown 3=4(5)-3=17$ so $(2 \blacktriangle (5 \blacktriangledown 3)) =2 \blacktriangle 17=2(2)+17=\boxed{21}$.